This invention relates to an ultrasonic welding device, particularly for welding together superposed films and comprises an elongated energy concentrating horn (hereafter "sonotrode").
Swiss Pat. No. 613,170 discloses vibration welding for welding a cardboard coated with a thermoplastic material. In this arrangement mechanical energy is transformed to local heat energy in order to reduce the required heat and energy quantities. Further, U.S. Pat. No. 3,526,792 discloses an ultrasonic transducer which comprises a converter for changing electrical energy into mechanical energy and a sonotrode, by means of which the mechanical energy is concentrated as an oscillating energy with a predetermined frequency of oscillation. In operation, the sonotrode cooperates with an anvil whereby the coated parts are pressed together between the work face of the sonotrode and the work face of the anvil. The oscillation of the sonotrode generates in the compressed workpiece frictional heat which is sufficient to activate the thermoplastic layers in such a manner that the thermoplastic material of the layers flow together. Upon stopping the oscillation, the thermoplastic material cools and solidifies and, as a result, the superposed layers are bonded to one another.
U.S. Pat. No. 4,279,675 describes an application of the above-outlined welding method in the packaging industry. The anvil is, at the edge of its work face and in a central portion thereof, provided with a cutout to accommodate the increased thicknesses of superimposed cardboard portions. Between the cutouts there are provided longitudinally arranged grooves to allow the liquefied thermoplastic material to be forced aside at the crests between adjoining grooves and to allow it to gather in the grooves to form a weld.
It has been found that the above-outlined arrangement is insufficient in case there are multilayer portions at some locations as it is the case, for example, in longitudinal seams of tubular foil wrappers. Thus, there may be locations which remain unwelded which would cause air leaks in an evacuated package.